A hybrid vehicle (HV) or a plug-in hybrid vehicle (PHV) normally uses a combination of an engine (internal-combustion engine) and an electric power as a power source. In the plug-in hybrid vehicle, a larger number of cells (secondary cells) are mounted as an assembled battery than mounted in the hybrid vehicle.
In the PHV, as shown in FIG. 8, the assembled battery is used such that partial charging/discharging is repeated in a wide range of the state of charge (SOC) of the battery capacity of the assembled battery, that is, extremely small SOC changes occur in various SOCs.
An assembled battery is made of a combination of a plurality of secondary cells. In the assembled battery, charging and discharging are simultaneously performed in the individual secondary cells. When the charging and discharging are repeated in each of the secondary cells as a result of the charging and discharging of the assembled battery, variations occur in the amounts of charge and SOCs of the secondary cells.
When there are variations in the SOCs of the secondary cells, the assembled battery cannot exhibit inherent battery performance. Specifically, when the assembled battery is charged (fully charged) on the basis of the lowest SOC among the SOCs of the secondary cells, the secondary cells other than the secondary cell having the lowest SOC are overcharged so that the SOCs of the secondary cells exceed 100%. Conversely, when the assembled battery is charged (fully charged) on the basis of the highest SOC among the SOCs of the secondary cells, the SOCs of the secondary cells other than the secondary cell having the highest SOC do not reach 100%, so the SOC of the assembled battery does not also reach 100%. When the charging/discharging is repeated in the assembled battery in which the secondary cells have variations in the SOCs, the variations in the SOCs of the secondary cells increase, so the degradation of the battery performance of the assembled battery accelerates.
When the charging/discharging of the assembled battery is further repeated in the state where there are variations in the amounts of charge of the secondary cells, the degradation of the assembled battery is accelerated.
Thus, in the assembled battery, the SOCs of the individual secondary cells need to be equalized and the SOCs need to be sensed for this purpose.
For sensing the SOC, various methods have been proposed. For example, a method which senses the SOC on the basis of a battery voltage is described in JP 2010-257984 A.